The present invention refers to a method and a device for firing solid fuels, substantially in the form of lumps or pieces, in a furnace having a combustion chamber with a substantially horizontal bottom, upon which is maintained a fuel bed with relatively great thickness while supplying fuel within the very bed, through which bed primary combustion air is fed from below through mutually spaced primary air passages in the combustion chamber bottom, the combustion gas which leaves the chamber furthermore being subjected to a subsequent secondary combustion in a secondary combustion chamber after addition of secondary combustion air.
Combustion methods and devices of this kind are previously known and it is also known to fire such furnaces with insufficient supply of primary air, which implies that above the fuel bed is formed an energy-rich gas which after admixing with secondary air is burnt in a secondary combustion chamber. A problem in connection with firing methods and devices of this kind resides in the removal of ash and slag from the combustion chamber. Conventionally, the bottom of the combustion chamber is constituted by a grid-like grate through which primary air is supplied. Ash and slag have to be removed by hand after stopping the fuel supply and complete down-burning of the fuel bed.
Another problem in connection with previously known firing methods and furnaces of the above-stated kind is that it is extremely difficult to obtain a uniform distribution of the primary air flow over the entire cross section of the combustion chamber. This leads to a non-uniform combustion and impaired efficiency.
Partly, the above-stated problems are dependent on each other since the formation of slag within one area of the combustion chamber cross section causes an impaired air flow within said area such that the air distribution becomes non-uniform. This causes an unsuitable combustion temperature and slag formation.